1. Field of the Invention
This invention relates to a 1-4-(alkylphenyl-ethynyl)-4-(alkylphenylethynyl)benzene as a substituted benzene derivative used as a component of liquid crystal materials and a liquid crystal composition containing the derivative.
2. Description of the Related Art
Display devices having applied liquid crystals utilize an electrooptical effect based on the anisotropies of the dielectric constant and electric conductivity of liquid crystal substances. Liquid crystal display modes include various ones such as dynamic scattering mode, twisted nematic mode, super-twist nematic mode, phase transition mode, DAP mode, guest-host mode, etc. Properties required for liquid crystal substances used for liquid crystal display vary depending on the respective liquid crystal display modes, but a broad mesomorphic range, good stabilities to moisture, air, light, electricity, etc. and the like are required in common to any of the display modes. Further, it is also required that when display elements are used in the liquid crystal display devices, the response of the display elements is quick and the devices can be driven at a voltage as low as possible. At present, however, there is no single compound satisfying all of these requirements; hence, practically, liquid crystal compositions obtained by blending several kinds of liquid crystal compounds or these compounds with compounds similar to liquid crystals have been used.
Recently, in order to provide a liquid crystal display element having a good image quality even in a multiplex number of 100 or more, it has been proposed to change the cell structure wherein the twist angle of the helical structure of liquid crystal molecule is combined with a polarizing plate, into a novel mode (e.g. Japanese patent application laid-open No. Sho 60-50511 (1985), etc.).
The case of a liquid crystal display element of a cell structure having an increased the twist angle of the liquid crystal molecule exhibits an entirely different tendency from that of the case of conventional 90.degree. twist, in the effect on physical properties obtained by choosing liquid crystal materials.
FIGS. 1a and 1b illustrate the characteristics of a liquid crystal display element having an increased twist angle of a liquid crystal molecule as compared with those of conventional 90.degree. twist, in terms of the angle of view and the angle dependency of voltage-transmittance characteristics. FIGS. 1a and 1b illustrate a case of 90.degree. twist and a case of 180.degree. twist, respectively. As seen from FIGS. 1a and 1b, the element having a structure of 180.degree. twist is steep in the rise characteristic (Y characteristic) of transmittance depending on voltage, and this characteristic is evidently improved as compared with the case of conventional 90.degree. twist. This tendency becomes more notable with increase of twist angle. As described above, since the element having an element structure increased in the twist angle is steep in the rise characteristic brought about by voltage, the transmittance difference between voltage impression and nonimpression at the time of multiplex drive increases so that a multiplex drive higher than a conventional one becomes possible.
However, as to the relationship between the .gamma. characteristic in the case of 90.degree. twist and that in the case of about 200.degree. twist, the same tendency is not exhibited depending upon liquid crystal materials; thus there has come to appear a tendency which does not apply to the general report that, in the case of 90.degree. twist, materials having a good .gamma. characteristic are superior when the ratio of elastic constants (K.sub.33 /K.sub.11) is low (Gunter Baur, Euro Display, 84 "Liquid crystal properties in relation to multiplexing requirements"). This is presumed to be related to the elastic constant of twist and other factors, but it has not yet been elucidated. FIG. 2 shows a relationship between the ratio of elastic constants and the .gamma. characteristic in the cases of 180.degree. twist and 230.degree. twist as to two kinds of compounds, i.e. pyrimidines and PCHs (phenylcyclohexanes). As seen from FIG. 2, pyrimidines having smaller elastic constants are inferior to PCHs in the .gamma. characteristic. In FIG. 2, .gamma. 10.degree., 80-20% refers to a .gamma. characteristic in terms of the ratio of a voltage at 80% transmittance to that at 20% transmittance in the case of an angle of 10.degree. against normal. As described above, in the case of about 200.degree. twist, a conventional way of thinking does not apply, and it is necessary to choose liquid crystal materials on a different basis from that in the case of the cell structure of 90.degree. twist. As described above, by increasing the twist angle or researching liquid crystal materials, a liquid crystal display element having a good .gamma. characteristic is obtained. However, a new phenomenon has been clarified by improving the .gamma. characteristic the .gamma. characteristic of liquid crystal materials, the lower the response rate, as shown in FIG. 3. When the response properties are taken into consideration, choice a of only a material having a good .gamma. characteristic is not sufficient, but in order to obtain a liquid crystal display element having good response properties, while maintaining .gamma. characteristic to a certain extent, a method of reducing the thickness of the liquid crystal layer is proposed.
Thus, with accompaniment of reducing the thickness of the liquid crystal layer in order to improve the response properties, the optical anisotropy value .DELTA.n of the material should be varied. In the case of 200.degree. C. twist, the product of the .DELTA.n of the liquid crystal material by the thickness d of the liquid crystal layer (.DELTA.n.times.d) is best in the vicinity of 0.96 .mu.m. When the thickness d of the liquid crystal layer is 7 .mu.m, the .DELTA.n of the material should be adjusted to 0.137. In order to make the thickness of the liquid crystal layer 5 .mu.m for improving the response rate, it is necessary to increase the .DELTA.n to 0.192. As described above, in order to correspond to thinning of the liquid crystal layer, the .DELTA.n of the material should be increased, but in this case, there is a problem of viscosity. As to the relationship between the .DELTA.n and viscosity of liquid crystal materials so far reported, a tendency that the viscosity increases with increase in the .DELTA.n has been clarified. Namely, conventional materials having a large .DELTA.n and yet a low viscosity have been very few.
As examples of so far known compounds having a large optical anisotropy value .DELTA.n when used as a component of liquid crystal materials, compounds expressed by the following formulas (1)-(4) are disclosed in (1) French patent application laid-open No. 2,141,438, (2) Japanese patent application laid-open No. Sho 60-152427/1985, (3) Japanese patent application laid-open No. Sho 61-260031/1986 and (4) Japanese patent application laid-open No. Sho 60-204731/1985, respectively: ##STR2## wherein R.sup.3 and R.sup.4 each represent an alkyl group or an alkoxy group; ##STR3## and R.sup.5 and R.sup.6 each represent a linear alkyl group; ##STR4## wherein R.sup.7 represents a linear alkyl group and X represents a halogen atom; and ##STR5## wherein R.sup.8 and R.sup.9 each represent a linear alkyl group.
These tolan derivatives have been said to have a large .DELTA.n and a relatively low viscosity, but compounds far exceeding the characteristics of these compounds have recently been required.